There is increasing demand not only for audio communication and data communication but also for multimedia communication in which moving pictures are transmitted as well as audio and data. As a means for realizing such broadband communication, an agreement has been reached upon an exchanging technique in a B-ISDN (Broadband-ISDN) system, which is based on an asynchronous transfer mode (ATM), and the technique is being put to practical use.
In the ATM, all the information is converted into fixed information which is called a cell without depending upon continuous information such as an audio and a moving picture or burst information such as data, and transferred at high speed without depending upon the respective communication speed. More specifically, in the ATM, a line is allocated to a plurality of calls by establishing a multiplicity of logical links on a physical line. The moving picture data and the audio data transmitted from a terminal corresponding to each call are separated into information units (which are called cells) having a fixed length, and the cells are serially transmitted over a line, thereby realizing multiplex communication.
Each cell is composed of a block having a fixed length of 53 bytes, as shown in FIG. 19. In the 53 bytes, 5 bytes constitute a header portion HD and 48 bytes an information field (information portion) DT. The header portion HD includes a virtual channel identifier (VCI) for identifying a call so as to indicate the destination even after the data is separated into blocks. The header portion HD also includes a virtual path identifier (VPI) for specifying a path, a generic flow control (GFC) which is used for flow control between links, a payload type (PT), a header error control (HEC) for correcting errors, etc.
(a) ATM network
FIG. 20 schematically shows the structure of an ATM network so as to explain ATM transmission. In FIG. 20, the reference numerals 1a, 1b represent ATM terminals, and 3 an ATM network. The ATM network 3 is provided with an information network 3a for transferring a data cell, and a signal network 3b for transferring a control signal. The call processing processor units (CPUs) 3d-1 to 3d-n of the ATM exchanges 3c-1 to 3c-n in the information network 3a are connected to the signal network 3b.
When the originating terminal 1a executes a calling operation so as to call the terminating terminal 1b, the cell assembling portion of the originating terminal separates the SET UP message (data which includes the originating number, the terminating number, the type of terminal, the average cell rate, the peak cell rate, etc.) into cell units, attaches a signal VCI (which is determined in advance for the respective terminal) to each divided data to form a signaling cell and supplies the signaling cell to the ATM network 3.
When the signaling device of the ATM exchange 3c-1 (on the originating side) receives the signaling cell, it assembles the information contained in the signaling cell and supplies the assembled information to the CPU 3d-1. On the basis of the received message, the CPU executes calling such processing as processing for analyzing calling-party service, billing processing, processing for interpreting digits on the side of the terminating party, etc., determines a path (VPI) and a virtual channel identifier (VCI), and in accordance with a protocol No. 7, supplies connection information which includes data such as the originating number, terminating number, VPI and VCI, to the subsequent relay exchange 3c-2 via the signal network 3b. The relay exchange 3c-2 executes similar processing to that of the exchange 3c-1 on the originating side. After repetition of similar processing, the paths and the relay ATM exchanges 3c-2, 3c3, . . . between the exchange 3c-1 on the originating side and the ATM exchange 3c-n to which the terminating terminal 1b is connected are finally determined. When the ATM exchange 3c-n on the terminating side receives the connection information including the originating number, the terminating number and the VCI of the higher-order ATM exchange 3c-3, the ATM exchange 3c-n allocates a predetermined VCI to the terminating terminal 1b and judges whether or not the terminating terminal 1b is communicable. If the answer is YES, the signal network 3b informs the exchange 3c-1 on the originating side that the terminating terminal 1b is communicable, and the exchange 3c-1 on the originating side allocates a predetermined VCI to the originating terminal 1a.
Each of the ATM exchanges 3c-1 to 3c-n on each path registers the following into an internal routing table in a form correlated with the VCI of the higher-order ATM exchange:
(1) a tag which includes the routing information for specifying outgoing highway (output link) of the cell having the particular VCI and the information for maintaining the quality, and
(2) new VCI and VPI which are to be added to the output cell in place of the old VCI and VPI.
In this manner, when the paths are formed between the originating terminal 1a and the terminating terminal 1b, these terminals 1a, 1b transmit and receive the call cells and the response cells and confirm the communication procedure in mutual fashion. Thereafter, the originating terminal 1aseparates the data to be transmitted into predetermined byte lengths, generates a cell with a header including the allocated VCI attached thereto, and supplies the cell to the ATM network 3. When the cell is input from the higher-order exchange through a predetermined incoming highway, each of the ATM exchanges 3c-1 to 3c-n replaces the VPI/VCI of the input cell by reference to its routing table and sends the cell out on a predetermined outgoing highway on the basis of the tag (routing information). As a result, the cell output from the originating terminal 1a reaches to the exchange 3c-n on the terminating side via the paths determined by the call control. The exchange 3c-n on the terminating side replaces the VCI which is attached to the input cell with the VCI allocated to the terminating terminal 1b by reference to its routing table and supplies the cell to the line to which the terminating terminal 1b is connected.
Thereafter, the originating terminal 1a serially transmits the cells to the terminating terminal 1b, and the terminating terminal 1b assembles the information portion DT contained in the received cells and restores the original data.
In the above explanation, only one call is processed, but by providing different VCI values for both ends of the respective lines between a terminal and an ATM exchange and between mutually adjacent ATM exchanges, it is possible to establish logical links on one line in correspondence with a multiplicity of calls, thereby realizing high-speed multiplex communication. According to the ATM, it is possible to multiplex information from information sources having different transmission speeds such as moving pictures, data and audio, so that a single transmission line can be effectively used. In addition, data transmission at a very high speed on the order of 150 Mbps to 600 Mbps is enabled without the need for retransmission control or a complicated communication procedure which is conventionally implemented by software through packet switching.
An ATM exchange has a buffering function, which enables the ATM exchange to accept a call without keeping the originating terminal waiting and to send it to the terminating terminal even if there are a multiplicity of calls to the ATM exchange or the terminating terminal. For example, when there are a multiplicity of simultaneous calls to the terminating terminal 1b and therefore there is no vacant line between the exchange 3c-n on the terminating side and the terminating terminal 1b, there remains a cell which cannot be sent to the terminating terminal 1b. In this case, the exchange 3c-n on the terminating side buffers the remaining cell and sends it to the terminating terminal 1b when a line becomes vacant. In this manner, it is possible to accept a call to the terminating terminal without keeping the originating terminal waiting.
(b) Self-routing ATM exchange
FIG. 21 shows the structure of a self-routing ATM exchange. The self-routing ATM exchange is provided with a basic switching unit SWU, a control information add-on unit CIAU, and a CPU (call controller) for processing a calling. Although one self-routing switch module SRM 1 exists between the input lines and the output lines in this ATM exchange, a plurality of self-routing switch modules may be connected between them.
The input ends of the module SRM 1 are connected to the input lines (input links) #1 to #3 via the control information add-on unit CIAU, and the output ends are connected to the output lines (output links) #1 to #3. The control information add-on unit CIAU is provided with add-on circuits AC1 to AC3 for adding routing information or the like in correspondence with the respective input lines #1 to #3. Each of the add-on circuits AC1 to AC3 adds a tag (routing header) to the cell which is input from the corresponding input line, replaces the VCI contained in the cell information and supplies the cell to the basic switching unit SWU.
The call controller CPU controls a call so as to determine the VCI and the VPI of a cell at the time of calling, determines the tag (routing header RH) in accordance with the location of the terminating terminal and writes the control information (VPI, VCI, RH) in a routing table (not shown) of the add-on circuit to which the cell is input. When the cell is input to a predetermined input line via the higher-order ATM exchange after the end of the call control, one of the add-on circuit AC1 to AC3 which is connected to the input line reads, from the routing table, the control information (tag and VCI) which corresponds to the VCI attached to the input cell. The add-on circuit adds the tag (routing header RH) to the cell, replaces the VCI of the cell with the read VCI, and supplies the cell to the basic switching unit SWU. The self-routing switch module SRM 1 of the basic switching unit SWU transmits the cells from a predetermined output line in accordance with the tag (routing header RH).
FIG. 22 is a circuit diagram of an example of the self-routing switch module (SRM 1). The symbols I.sub.1 to I.sub.3 represent control information detectors, D.sub.1 to D.sub.3 transmission information delay circuits, DM.sub.1 to DM.sub.3 demultiplexers, and DEC.sub.1 to DEC.sub.3 control information decoders. All these elements constitute a cell distributor CELD. The symbols FM.sub.11 to FM.sub.33 represent buffer memories such as FIFO (First-In First-Out) memories, SE.sub.L1 to SEL.sub.3 selectors, and AOM.sub.1 to AOM.sub.3 arrival-order management FIFOs. The arrival-order management FIFOs (AOM.sub.1 to AOM.sub.3), which are connected to the output ends of the control information decoders DEC.sub.1 to DEC.sub.3, respectively, store the order of arrival of the cells into the corresponding three buffer memories FM.sub.11 to FM.sub.13, FM.sub.21 to FM.sub.23 and FM.sub.31 to FM.sub.33, respectively, control the selectors SEL.sub.1 to SEL.sub.3, respectively, so as to read the cells from the buffer memories in the order of arrival, and supply the cells to the output lines #1 to #3, respectively.
The detector I.sub.i (i=1 to 3) extracts the control information contained in the cell and supplies the information to the decoder DEC.sub.i (i=1 to 3).
If the input tag (routing header RH) represents the output terminal #j (j=1 to 3), the decoder DEC.sub.i operates the demultiplexer DM.sub.i by a switch signal S.sub.i and transmits the transmission information to the FIFO memory FM.sub.ji. For example, if the routing header RH contained in the information input from the input terminal #1 represents the output terminal #2, the decoder DEC.sub.1 operates the demultiplexer DM.sub.1 and inputs the information supplied from the input terminal #1 to the FIFO memory FM.sub.21. The arrival-order management FIFO (AOM.sub.i) is connected to the output terminal of the corresponding control information decoder DEC.sub.1 to DEC.sub.3 and stores the order of arrival of the cells to the corresponding three buffer memories FM .sub.i1 to FM.sub.i3. For example, if the cells arrive to the buffer memories in the order of FM.sub.11, FM.sub.12, FM.sub.13, FM.sub.12, . . . , buffer memory identification codes 1, 2, 3, 2, . . . are stored in the arrival-order management FIFO (AOM.sub.i) in the order of arrival of the cells. Thereafter, the arrival-order management FIFO (AOM.sub.i) controls the corresponding selector SEL.sub.i to read the cells from the three buffer memories FM.sub.i1 to FM.sub.i3 in the order of arrival of the cells and supplies the cells to the output line #i.
In this manner, since the FIFO memory FM.sub.ij has a capacity for a plurality of cells, it has a buffering function which is capable of adequately dealing with the problem such as a temporary increase of transmission data. In addition, since cells are read from the buffer memories FM.sub.i1 to FM.sub.i3 in the order of arrival of the cells, an equal number of cells remain in each of the buffer memories FM.sub.i1 to FM.sub.i3, and it never happens that cells overflow a buffer memory and are therefore discarded.
The ATM transmission system, however, has the following problem. Since various traffics having different information speeds and different burst properties (burst means an abrupt increase in the quantity of information) are synthetically handled in the ATM transmission system, in a case where there is a traffic having an especially strong burst property, it is impossible without an appropriate call reception control to transmit the ATM cells so as to satisfy a QOS (cell loss ratio, delay time) which is required by the user. For this reason, when a bandwidth-guaranteed connection call generates, an ATM exchange judges whether or not there is a vacant bandwidth which the call requires in a predetermined transmission line on the basis of the physical bandwidth of the transmission line and the average and peak cell rates of the call declared by the user (ATM terminal), and if the answer is in the affirmative, the ATM exchange accepts the call, while rejecting the call if the answer is in the negative.
There are two methods for determining whether the ATM exchange accepts or rejects a call having a variable-speed traffic property in which the average cell rate is different from the peak cell rate. In the first method, the ATM exchange determines whether or not the call is accepted by regarding the peak cell rate of the call as a necessary bandwidth. The first method is simple, but it reduces the number of calls which can be allocated to a transmission line, thereby lowering the utilization of a transmission line. In the second method, the ATM exchange determines whether or not the call is accepted by regarding the average cell rate of the call as a necessary bandwidth. According to the second method, many calls can be allocated to a transmission line, thereby enhancing the utilization of a transmission line. However, when the peak of the transmission rate for each call overlaps each other, cells beyond the physical bandwidth of the transmission line are lost. As a result, it is impossible to meet the required cell loss ratio, which causes sound skipping, picture missing and data loss on the terminating side. In order to solve these problems, cells are allocated both on the basis of average cell rate and on the basis of the peak cell rate in the call reception control adopted at present, thereby enhancing the utilization of a transmission line wile maintaining a predetermined cell loss ratio.
FIG. 23 is an explanatory view of the call reception control in an ATM exchange. In FIG. 23, (1) the reference symbol Vt represents the physical bandwidth of a transmission line, (2) Vpht the sum of the peak cell rates of all the calls allocated on the basis of the peak cell rate, (3) Vavt the sum of the average cell rates of all the calls allocated on the basis of the average cell rate, and (4) Vpts the sum of the peak cell rates of all the cells that are in the process of communication. Further, (5) the reference symbol Vp represents the peak cell rate of a new call (which is requesting admission), and (6) Vav the average cell rate of the call which is requesting admission.
When the number of calls allocated to the transmission line increases, the peaks and the bottoms of the transmission rates overlap each other and they are levelled due to a statistical multiplexing efficiency, so that it is possible to accommodate a larger number calls than an apparent number of calls. In call reception control referred to as connection admission control (CAC), cells are allocated on the basis of both the average cell rate and the peak cell rate by utilizing such a statistical multiplexing efficiency.
(c) Statistical multiplex system
In an ATM exchange, the ATM cells which arrive from a plurality of (N) input links are concentrated and multiplexed before being transmitted from a predetermined output link, as shown in FIG. 24. As such a concentration multiplex system, there are 1 a cross-point buffer system by skip polling control, 2 a cross-point buffer system by FIFO read control, 3 a cross-point buffer system by LNQ priority read control, and 4 an output buffer system.
In the cross-point buffer system by the skip polling control, the buffers (buffers FM11 to FM33 shown in FIG. 22) are serially scanned and ATM cells are transmitted, while a vacant buffer is skipped. In the cross-point buffer system by the FIFO read control, the order of arrival of the ATM cells from all input links is collectively managed and the ATM cells are serially transmitted in the order of arrival, as explained in FIG. 22. In the cross-point buffer system by the LNQ priority read control, an ATM cell is read preferentially from the buffer in which the largest number of cells are stored. The LNQ is an abbreviation of Largest Number of Cells in the Queue. These cross-point buffer systems are premised on that scanning of all the buffers or comparison of the number of cells stored in the buffers can be executed within one cell reading time.
In the output buffer system, cells are multiplexed so that the speed is temporarily raised to N.multidot.V in the ATM exchange, thereafter the speed N.multidot.V is converted to the speed V (N.multidot.V.fwdarw.V) by a single FIFO. FIG. 25 explains such an output buffer system. The symbol MPX represents a multiplexer, and DBF.sub.1 to DBF.sub.N FIFO memories as output buffers. The multiplexer MPX multiplexes the cells for output lines #1 to #N having a speed of V, and stores the multiplexed cells in the output buffers DBF.sub.1 to DBF.sub.N provided in correspondence with the respective output lines #1 to #N at a speed of NV. Then, the cells are read from the respective buffers at the speed of V and output to the corresponding output lines #1 to #N.
These concentration multiplex systems 1.about.4 will be compared with each other in the following.
(1) In regard to the buffer lengths required in the case of random traffic input, the systems 4, 3, 2, 1 require a shorter buffer length in that order.
(2) With the fact that the output buffer system 4 requires a high-speed memory having a speed of N times of V taken into consideration, the memory costs in each system is represented by the following formula, EQU memory cost=.sqroot.(N.multidot.buffer length).
Therefore, in regard to the memory costs, the systems 3, 2, 4, 1 require a lower cost in that order.
(3) In the case of burst input, there is a knee point at which the cell loss ratio is not reduced however longer the buffer length may be. Due to this influence, there is not much difference as to the cell loss ratio if the buffer length is long. FIG. 26 shows the relationship between the logarithm of the cell loss ratio represented by logcell loss ratio! and the buffer length, wherein the symbol n represents the number of multiplexed calls, and Vout the speed of a transmission line. As is clear from FIG. 26, the larger the number n of multiplexed calls becomes, the knee point moves downward, so that the cell loss ratio is reduced. This is because the larger the number n of multiplexed calls becomes, the more peaks and bottoms of the transmission speeds for the multiplexed calls overlap with each other due to a statistical multiplexing efficiency thereby the transmission speed of each call is leveled.
(4) In order to lower the knee point in the case of burst input, it is necessary to increase the transmission speed at the output link in comparison to the peak cell rate, which leads to an increase in the multiplex degree.
In the future, the peak cell rate of the calling source itself will increase. This means that in order to lower the knee point, it is necessary to enhance the speed of the resource of a network for multiplexing the calls. Therefore, the output buffer system 4 which is primarily required to memorize the cells at high speed and therefore has no margin in the speed should not be adopted.
Although the cross-point buffer system by the LNQ priority read control 3 is superior to any other cross-point buffer system in the point of the cell loss ratio, it should not be adopted either because it has the following defects;
the embodiment of the mechanism itself is difficult, and
since a buffer in which the smallest number of cells are stored has a possibility of continuously losing the priority and the delay time of the cell may be increased, control using a threshold is essential.
Consequently, the cross-point buffer system by FIFO read control is the most realistic as a system for transmitting the ATM cells queued in buffers to an output link.
When such statistical multiplex is executed, the utilization of an output link is enhanced. However, there is a possibility of one connection continuously occupying the bandwidth of the output link, and it is impossible to desirably allocate a bandwidth for each connection or each input link.
(d) Traffic shaping system
In contrast to the statistical multiplex system, there is a traffic shaping system as a method of desirably allocating a bandwidth for each connection or each input link. FIG. 27 shows the structure of a traffic shaping system. In the traffic shaping system, the physical bandwidth V (e.g., V=150 Mb/s) of one output link is divided suitably and each divided bandwidth is allocated in advance to each input link, and the ATM cells input from each input link is transmitted to the output link so as to satisfy the allocated bandwidth. According to the traffic shaping system, by managing the bandwidth to be used in the output link for each input link, it is possible to prevent a phenomenon that such a greater part of the bandwidth of the output link is occupied by a specific connection as to make it impossible to transmit the cells of the other connections. In FIG. 27, the symbol CELD represents a cell distributor (see CELD in FIG. 22), FM11 to FM1n FIFO buffers provided in correspondence with input links, SEL a selector, and a SCD a scheduler for reading an ATM cell from each buffer in accordance with a preset schedule and transmitting it to the output link #1. In the scheduling table within the scheduler SCD, for example, one period is divided into 128 cell slots (time slots) and each of the cell slots S.sub.1 to S.sub.128 is allocated to a buffer (input link) from which cells are read and supplied to the output link. Accordingly, at every one of the 128 cell slots of one period, the scheduler reads a cell from a predetermined buffer and supplies it to the output link. This operation is conducted cyclically. By this operation, a bandwidth V1, V2, . . . Vn (V1+V2+ . . . Vn.ltoreq.V) is allocated to each input link. When the number of cells supplied from a buffer in one period is a plural, it is preferable to make the interval between cells as uniform as possible.
As described above, according to a traffic shaping system, the bandwidth allocated to each input link becomes desirable. However, in contrast to a statistical multiplex system, even if there is a buffer in which more cells arrive and an output link is vacant, it is inconveniently impossible to read a cell from the buffer and supply it to the output link until the time for accessing the buffer comes. In other words, the utilization of an output link is not very high in a traffic shaping system.
(e) Congestion
In an ATM network, cell loss, that is, a loss of transmission information is caused by simultaneous arrival of cells to a buffer (short time congestion). In the case of relaying between high-speed data communication networks such as frame relay networks by an ATM network, a frame, which is an information transmission unit of a high-speed data communication network, sometimes becomes incomplete (an error frame is generated) due to the cell loss in the ATM network. The error frame is complemented by retransmission control in dependence upon the higher-order protocols of the terminal. At this time, there is a fear of an increase in the load on the ATM network, which furthers and deteriorates the congestion in the ATM network, resulting in a long-term congestion, depending upon the connection form between the high-speed data communication network and the ATM network or the retransmission control of the higher-order protocols.
FIG. 28 is an explanatory view of a short-time congestion and a long-term congestion. The abscissa represents time and the ordinate the length (queue length) of the cells stored in a queue in a buffer. The symbol CTH represents a congestion threshold for detecting a congestion. A short-term congestion SCJ is caused when the buffer queue length is momentarily increased, and it is autonomously relieved. On the other hand, in a long-term congestion LCJ, the buffer queue length is held at its maximum. When the congestion threshold CTH is equal to the buffer size, all the cells above the dot line are lost.
FIGS. 29 and 30 are explanatory views of retransmission control, wherein FIG. 29 shows retransmission control when there is no frame loss, and FIG. 30 shows retransmission control when there is frame loss. In these drawings, the retransmission process of data communication is exemplified by a Go-Back-N process (HLDC process), which is the most general as a data communication retransmission process. In the Go-Back-N process, after a sequence number is attached to each frame, flow control of each frame or retransmission control is executed. Each frame is provided with a field for recording the sequence number of a received frame so as to acknowledge the arrival of the frame (acknowledge normal communication). More specifically, the terminating side records the sequence number of the latest one of the consecutively normally received frames in a frame which is transmitted in the reverse direction (from the terminating side to the originating side) and acknowledges the arrival of the frames between the terminating side and the originating side (see ACK (2) and ACK (4) in FIG. 29). A frame loss is detected from the discontinuity of sequence numbers of the received frames (see FIG. 30). Similarly, it is general that the sequence number of a lost frame is notified to the originating side by a frame transmitted in the reverse direction (see REJ (2), the numeral 2 is a sequence number of the lost frame).
In the Go-Back-N process, the originating which has received the report REJ (2) of the sequence number of the lost frame retransmits all the frames subsequent to the lost frame which is indicated by the supplied lost frame sequence number (=2), as shown in FIG. 30. This is because the terminating side regards all the transmitted frames subsequent to the lost frame as invalid, and discards them. Consequently, even if there is only one frame loss, the number of retransmitted frames is a plural, so that when a plurality of lines execute a retransmission process, the congestion is deteriorated due to an increase in the load, which may result in a long-term congestion.
(f) Problems in statistical multiplex system and traffic shaping system and objects of the invention
According to a statistical multiplex system, the utilization of an output link is enhanced. However, there is a possibility of one connection continuously occupying the bandwidth of the output link, and it is impossible to desirably allocate a bandwidth for each connection or each input link. In contrast, in a traffic shaping system, although it is possible to desirably allocate a bandwidth for each input link, the utilization of an output link is not very high. In this way, both statistical multiplex system and a traffic shaping system have respective merits and defects. An ATM exchange having the merits of both system by utilizing both systems is therefore demanded.
There are two types of call; one is a bandwidth-guaranteed connection call which requires a bandwidth to be guaranteed, and the other is a non-bandwidth-guaranteed connection call which does not require a bandwidth to be guaranteed and a cell of which may be transmitted only when no information of a bandwidth-guaranteed connection call is transmitted. When there are these two types of call together, it is desirable to decisively allocate the required bandwidth to the bandwidth-guaranteed connection call and to allow the non-bandwidth-guaranteed connection call to effectively use the remaining bandwidth.
Accordingly, it is a first object of the present invention to provide an ATM exchange which utilizes both a statistical multiplex system and a traffic shaping system to have the respective merits.
It is a second object of the present invention to provide an ATM exchange for transmitting the cell of a bandwidth-guaranteed connection call to an output link in accordance with a traffic shaping system and for transmitting the cell of a non-bandwidth-guaranteed connection call to the output link in accordance with a statistical multiplex system, for example, a cross-point buffer system by FIFO read control when there are simultaneously both the bandwidth-guaranteed connection call and the non-bandwidth-guaranteed connection call.
It is a third object of the present invention to provide an ATM exchange which is capable of decisively allocating a bandwidth to each bandwidth-guaranteed connection call.
It is a fourth object of the present invention to provide an ATM exchange which is capable of regulating the bandwidth to be allocated to a non-bandwidth-guaranteed connection call.
It is a fifth object of the present invention to provide an ATM exchange which is capable of decisively allocating a bandwidth to a bandwidth-guaranteed connection call, wherein the bandwidth is necessary for satisfying the quality specified by a quality class to which the bandwidth-guaranteed connection call belongs.
It is a sixth object of the present invention to provide an ATM exchange which is capable of acquiring a necessary bandwidth for a bandwidth-guaranteed connection call even if ATM cells of a non-bandwidth-guaranteed connection call frequently reach.
It is a seventh object of the present invention to provide an ATM exchange which is capable of acquiring a bandwidth so as to securely maintain the qualities of all the quality classes even if many ATM cells of a specific quality class reach, by inhibiting a bandwidth beyond the bandwidth in conformity with cell loss ratio which is specified for each quality class from being allocated to each quality class.
(g) Problems of long-term congestion and objects of the invention
Although a short-term congestion is autonomously relieved, the buffer queue length is held at its maximum and cell loss is caused for a long term in a long-term congestion. It is therefore necessary to exert congestion control so as to avoid a long-term congestion when the long-term congestion is detected and to release the congestion avoiding control when a normal state is restored.
As a method of detecting a congestion, there is a system of detecting a congestion by using the number of lost cells or a cell loss ratio. This system is advantageous in that it does not depend upon the form of the information which flows into a buffer, but it is defective in that since it requires a special hardware for observing the cells lost for a short period, it increases a hardware in an ATM exchange and therefore makes the ATM exchange complicated.
As a system which is realized by a simple hardware, there is a system of setting a threshold of the buffer queue length and detecting and judging a congestion merely by taking whether the buffer queue length becomes lager than the threshold of the buffer queue length into consideration. In this simple system, however, there is a difficult problem. That is, it is necessary to take the form of information which flows into the buffer into consideration when the threshold is determined. More specifically, if the information which flows into the buffer has a strong burst property, although the average queue length is small, the fluctuation (dispersion) in the vicinity of the average value is large, so that there is a case where the queue length momentarily exceeds the threshold, which inconveniently leads to excessive and frequent the congestion avoiding control.
It is therefore conventionally impossible to securely detect a long-term congestion discriminated from a short-term congestion by a simple structure.
Accordingly, it is an eighth object of the present invention to provide an ATM exchange which is capable of securely detecting a long-term congestion discriminated from a short-term congestion by a simple structure and starting congestion avoiding control.
It is a ninth object of the present invention to provide an ATM exchange which is capable of securely detecting that the ATM exchange network has restored a normal state from a long-term congestion so as to quickly release congestion avoiding control.
It is a tenth object of the present invention to provide an ATM exchange which is capable of obviating the difficulty in determining a threshold with the form of information flowing into a buffer taken into consideration.
It is an eleventh object of the present invention to provide an ATM exchange which is capable of avoiding an oscillation phenomenon that the operation of starting and ending congestion avoiding control is frequently repeated.